THERMAL AND PHOTOCHEMICAL OXIDATIVE ADDITION OF ALKYL-HALIDES TO THE CYCLOMETALATED COMPLEX CIS-BIS[2-(2'-THIENYL)PYRIDINE]PLATINUM(II)

The homoleptic, cyclometalated complex cis-bis[2-(2'-thienyl)pyridine] platinum(II) (1, Pt(2'-thpy)2) undergoes thermal or photochemical oxi dative addition (TOA or POA) reactions with a wide range of alkyl hali des. These reactions are found to be highly stereoselective, yielding in most cases Pt(IV) complexes with the halide and the sigma-bonded C( alkyl) atom in the cis position, in a C,C,C-fac arrangement. In some c ases, trans addition of R-X was observed, followed by trans double lin e arrow pointing right cis isomerization, either at room temperature o r on heating. Extensive use was made of H-1 NMR spectroscopy in assign ing the configurations, and the 2-D COSY spectra, measured for a few o f the Pt(IV) complexes, were helpful in making the chemical shift assi gnment. The Pt-195 NMR spectra were recorded for five of the Pt(IV) co mplexes. The resonance of the Pt(IV) nucleus is seen to be very sensit ive to the nature of the coordinated halide and alkyl ligands. The X-r ay crystal structures were determined for three of the Pt(IV) complexe s. Crystals of Pt(2'-thpy)2(benzyl)Br (8) are triclinic, space group P 1BAR, a = 9.854(2) angstrom, b = 15.413(2) angstrom, c = 17.333(2) ang strom, a = 63.90(1)-degrees, beta = 76.71(1)-degrees, gamma = 86.42(1) -degrees, and Z = 4. Final R = 0.030 and R(W) = 0.048 for 7758 observe d reflections. Crystals of Pt(2'-thpy)2(1-naphthyl)Br (12) are also tr iclinic, space group P1BAR, a = 7.335(2) angstrom, b = 10.484(2) angst rom, c = 16.912(2) angstrom, alpha = 84.02(1)-degrees, beta = 83.46(1) -degrees, gamma = 87.18(2)-degrees, and Z = 2. Final R = 0.044 and R(W ) = 0.065 for 2966 observed reflections. Both these structures reveal the cis addition of R-X and the distorted octahedral coordination sphe re of the Pt(IV) atom. Crystals of Pt(2'-thpy)2(CF3CHCl)Br (20a) are o rthorhombic, space group Pbca, a = 17.574(3) angstrom, b = 9.204(1) an gstrom, c = 26.612(2) angstrom, and Z = 8. Final R = 0.036 and R(W) = 0.068 for 2462 observed reflections. The structure shows the trans add ition of CF3CHBrCl. The one Pt-N bond is 0.08 angstrom longer than the other Pt-N bond in 20a, as well as the Pt-N bonds in 8 and 12. This i s probably due to steric interaction between the ''coplanar'' 2-thieny lpyridine ligands in this isomer.